[单选题]
A simple pendulum ex) 6yscgcr0o 3zo:wy3t ecuting simple 62n w(um*tbs aharmonic motion is released from one end and reaches equilibrium pos2tmu w n*b6(asition in $2\,s$. Just after the equilibrium position, the length of the pendulum is reduced to one-fourth of its initial value because of wrapping around a nail.
